Draft version September 10, 2021Typeset using LATEX twocolumn style in AASTeX63

TOI-532b: The Habitable-zone Planet Finder confirms a Large Super Neptune in the Neptune Desert

orbiting a metal-rich M dwarf host

Shubham Kanodia,1, 2 Gudmundur Stefansson,3, 4 Caleb I. Canas,5, 1, 2 Marissa Maney,1, 2 Andrea S.J. Lin,1, 2

Joe P. Ninan,1, 2 Sinclaire Jones,4 Andrew Monson,1, 2 Brock A. Parker,6 Henry A. Kobulnicky,6

Jason Rothenberg,6 Corey Beard,7 Jack Lubin,8 Paul Robertson,8 Arvind F. Gupta,1, 2 Suvrath Mahadevan,1, 2

William D. Cochran,9, 10 Chad F. Bender,11 Scott A. Diddams,12, 13 Connor Fredrick,12, 13 Samuel Halverson,14

Suzanne Hawley,15 Fred Hearty,1, 2 Leslie Hebb,16 Ravi Kopparapu,17, 18 Andrew J. Metcalf,19, 20, 21

Lawrence W. Ramsey,1, 2 Arpita Roy,22, 23 Christian Schwab,24 Maria Schutte,25 Ryan C. Terrien,26

John Wisniewski,25 and Jason T. Wright1, 2, 27

1Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802, USA2Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802,

USA3Henry Norris Russell Fellow

4Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA5NASA Earth and Space Science Fellow

6Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82070, USA7Department of Physics and Astronomy, The University of California, Irvine, Irvine, CA 92697, USA

8Department of Physics & Astronomy, University of California Irvine, Irvine, CA 92697, USA9McDonald Observatory and Department of Astronomy, The University of Texas at Austin

10Center for Planetary Systems Habitability, The University of Texas at Austin11Steward Observatory, The University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA

12Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA13Department of Physics, University of Colorado, 2000 Colorado Avenue, Boulder, CO 80309, USA

14Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA15Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195 USA

16Department of Physics, Hobart and William Smith Colleges, 300 Pulteney Street, Geneva, NY, 14456, USA17NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA

18Sellers Exoplanet Environment Collaboration (SEEC), NASA Goddard Space Flight Center19Space Vehicles Directorate, Air Force Research Laboratory, 3550 Aberdeen Ave. SE, Kirtland AFB, NM 87117, USA

20Time and Frequency Division, National Institute of Technology, 325 Broadway, Boulder, CO 80305, USA21Department of Physics, 390 UCB, University of Colorado Boulder, Boulder, CO 80309, USA

22Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA23Department of Physics and Astronomy, Johns Hopkins University, 3400 N Charles St, Baltimore, MD 21218, USA24Department of Physics and Astronomy, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia

25Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks Street, Norman, OK 73019, USA26Department of Physics and Astronomy, Carleton College, One North College Street, Northfield, MN 55057, USA

27Penn State Extraterrestrial Intelligence Center, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA,16802, USA

(Received June 16, 2021; Accepted July 28, 2021)

ABSTRACT

We confirm the planetary nature of TOI-532b, using a combination of precise near-infrared radial

velocities with the Habitable-zone Planet Finder, TESS light curves, ground based photometric follow-

up, and high-contrast imaging. TOI-532 is a faint (J∼ 11.5) metal-rich M dwarf with Teff = 3957± 69

K and [Fe/H] = 0.38 ± 0.04; it hosts a transiting gaseous planet with a period of ∼ 2.3 days. Joint

fitting of the radial velocities with the TESS and ground-based transits reveal a planet with radius of

Corresponding author: Shubham Kanodia

shbhuk@gmail.com

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2 Kanodia et al. 2021.

5.82 ± 0.19 R⊕, and a mass of 61.5+9.7−9.3 M⊕. TOI-532b is the largest and most massive super Neptune

detected around an M dwarf with both mass and radius measurements, and it bridges the gap between

the Neptune-sized planets and the heavier Jovian planets known to orbit M dwarfs. It also follows the

previously noted trend between gas giants and host star metallicity for M dwarf planets. In addition, it

is situated at the edge of the Neptune desert in the Radius–Insolation plane, helping place constraints

on the mechanisms responsible for sculpting this region of planetary parameter space.

Keywords: planets and satellites: detection, composition; planetary systems; stars: fundamental pa-

rameters; methods: statistical;

1. INTRODUCTION

Studies analyzing the host star metallicity dependence

of gas giant (Rp > 4 R⊕) occurrence rates have tradi-

tionally relied on a sample of planets orbiting solar type

stars, with a typical minimum photospheric temperature

corresponding to mid K dwarfs. Extending this analysis

to M dwarf planets has been hampered by the intrinsic

faintness of M dwarfs, which makes planet detection and

mass measurement difficult. Occurrence rate studies for

transiting planets orbiting M dwarfs have been limited

to the smaller (Rp < 4 R⊕) planets (Laughlin et al.

2004; Johnson & Apps 2009; Gaidos et al. 2013; Dress-

ing & Charbonneau 2015; Hsu et al. 2020). Attempts

to study the occurrence rates of gas giants orbiting M

dwarfs have used samples from radial velocity (RV) sur-

veys (Johnson & Apps 2009; Johnson et al. 2010; Gaidos

et al. 2013; Tuomi et al. 2019). Most recently, Maldon-

ado et al. (2020) use a sample of RV planets detected

from the HARPS-N spectrograph to probe the depen-

dence of gas giant occurrence on metallicity. Occur-

rence rate studies for gaseous planets using RV surveys

can be complicated by the lack of true mass measure-

ments (Mp vs Mp sini). Therefore, in its all-sky survey

of transiting planets around nearby-stars—and with its

red-optimized band-pass yielding high precision photo-

metric observations of nearby M-dwarfs— the Transiting

Exoplanet Survey Satellite (TESS; Ricker et al. 2014)

presents a unique opportunity to find transiting gas gi-

ants orbiting M dwarfs suitable for mass measurements.

Four such recent discoveries by TESS are—TOI-1728b

(Kanodia et al. 2020), TOI-1899b (Canas et al. 2020),

TOI-442b (Dreizler et al. 2020), and TOI-674b (Murgas

et al. 2021).

Transiting Neptune-sized planets (2R⊕ < Rp <

6R⊕)1, present a transitional population between rocky

terrestrial planets and Jovian gas giants. In particu-

lar, transiting super Neptunes (17M⊕ < Mp < 57M⊕;

Bakos et al. 2015), can help inform theories of planet

1 Also referred to as sub-Saturns (Petigura et al. 2018; Kopparapuet al. 2018).

formation and migration, i.e., did the gaseous giants

form in-situ close to their host star, or form away be-

yond the ice line and migrate inwards due to eccentric-

ity driven excitation or disk migration (Madhusudhan

et al. 2017; Bean et al. 2021; Fortney et al. 2021). This

investigation into the provenance of gaseous giants can

be further aided by atmospheric characterization using

transmission spectroscopy (Guzman-Mesa et al. 2020),

where the “warm Neptunes” with equilibrium temper-

atures between ∼ 800-1200 K, are expected to exhibit

diverse atmospheric elemental abundances, with possi-

ble imprints of the protoplanetary disk chemistry (Mor-

dasini et al. 2016).

Additionally, as predicted by Ida & Lin (2004a), Szabo

& Kiss (2011) and Mazeh et al. (2016) have noted a

dearth of Neptune-sized objects orbiting close to their

host star (2-4 day orbital period), referred to as the

“Neptune Desert”. Different hypotheses have been pro-

posed as a possible explanation to this feature, since

it can not be explained by observational biases. Mat-

sakos & Konigl (2016) attempt to explain the origin of

the Neptune Desert using high eccentricity migration,

whereas Owen & Lai (2018) show that photoevapora-

tion can be a driving factor responsible for the lower

boundary of the desert.

In this manuscript, we report the discovery of the

transiting Super Neptune TOI-532b using precision RVs

from the near infrared (NIR) Habitable-zone Planet

Finder spectrograph (HPF; Mahadevan et al. 2012,

2014), to measure the mass of a transiting super Nep-

tune orbiting the early type metal-rich M dwarf TOI-

532 in the constellation of Orion. We perform a com-

prehensive characterization of the stellar and plane-

tary properties using space-based photometric observa-

tions from TESS, additional ground-based transit ob-

servations, adaptive optics imaging, and high-contrast

speckle imaging. This paper is structured as follows. In

Section 2, we discuss the observations of this system,

which include space-based TESS photometry, ground-

based photometry, high contrast imaging, as well as

precision RV observations with HPF. In Section 3 we

discuss our characterization of the stellar parameters,

A super Neptune orbiting TOI-532 3

followed by Section 4, where we detail our joint analy-

sis of the photometry and velocimetry to constrain the

planetary parameters of TOI-532b. In Section 5, we

compare the properties of TOI-532b with other M dwarf

exoplanets, and with few other Neptunes to place it in

context for potential He 10830 A absorption detection

using transmission spectroscopy. Finally, we summarize

our results in Section 6.

2. OBSERVATIONS

2.1. TESS

TOI-532 (TIC-144700903, 2MASS J05401918+1133463,

Gaia EDR3 3340265717587057536, UCAC4 508-014156)

was observed by TESS in Sector 6 in Camera 1 from

2018 December 11 to 2019 January 7th at two minute

cadence (Figure 2). The Science Processing Operations

Center (SPOC) at NASA Ames (Jenkins et al. 2016)

reported one transiting planet candidate, TOI-532.01,

with a period of 2.326811 days. For our subsequent

analysis, we use the Presearch Data Conditioning Sin-

gle Aperture Photometry (PDCSAP) lightcurve, which

contains systematics and dilution corrected data us-

ing the algorithms originally developed for the Kepler

data analysis pipeline. We retrieved the data using the

lightkurve package (Lightkurve Collaboration et al.

2018), available at the Mikulski Archive for Space Tele-

scopes (MAST).

Figure 1 presents a comparison of the region contained

within the Sector 6 footprint from the Palomar Observa-

tory Sky Survey (POSS-1; Harrington 1952; Minkowski

& Abell 1963) image in 1951 and a more recent ZTF

(Masci et al. 2019) image from 2019. There are no bright

targets with ∆ GRP < 3 present in the TESS aperture,

however there are a few targets with ∆ GRP < 4, that

dilute the TESS transit. Even though this is taken into

account in the PDCSAP flux, following the methodology

of Burt et al. (2020) we use our ground based photome-

try to estimate an additional correction to this dilution

term photometry and discuss this in Section 4.

2.2. Ground Based Photometric Follow up

We obtain follow up transits from the ground, to val-

idate the transit seen in the TESS photometry, and

measure the dilution present therein. Furthermore, the

ground based photometry helps in improving the ra-

dius estimates as well as the ephemeris. These obser-

vations were pipeline processed using standard linear-

ity, bias, dark, and flat field corrections. We then per-

formed aperture photometry using AstroImageJ (Collins

et al. 2017). Clear outliers due to cosmic rays, charged-

particle events, poor seeing conditions, or telescope

tracking were removed using AstroImageJ. We experi-

mented using a number of different aperture settings,

and varied the radii of the photometric aperture, as well

as the inner, and outer background annuli, and selected

the settings that resulted in the minimum scatter in

the resulting photometry. Following the methodology

in Stefansson et al. (2017), we added the scintillation

error estimates to the photometric error estimated by

AstroImageJ. See Figure 3, and Table 1 for a summary

of all our ground based photometric observations.

2.2.1. RBO

We observed a transit of TOI-532b on the night of

2020 December 7 using the 0.6 m telescope at the Red

Buttes Observatory (RBO) in Wyoming (Kasper et al.

2016). The RBO telescope is a f/8.43 Ritchey-Chretien

Cassegrain constructed by DFM Engineering, Inc. It is

currently equipped with an Apogee ASPEN CG47 cam-

era.

The target rose from an airmass of 1.61 at the start

of the observations to a minimum airmass of 1.15 and

then set to an airmass of 1.20 at the end of the obser-

vations. Observations were performed using the Bessell

I filter (Bessell 1990) with 1 × 1 on-chip binning. To

prevent saturation, we defocused moderately (Table 1),

which allowed us to use an exposure time of 120 s. In

the 1× 1 binning mode, the 0.6 m at RBO has a gain of

1.27 e/ADU, a plate scale of 0.532′′, and a readout time

of approximately 2.4 s.

Due to cloud contamination, only the transit ingress

was recovered from these observations (Figure 3b). For

the final reduction, we selected a photometric aperture

of 17 pixels (9.04′′) with an inner sky annulus of 40 pixels

(21.3′′) and outer sky annulus of 60 pixels (31.9′′).

2.2.2. TMMT

We observed two transits of TOI-532b on the nights of

2020 December 15 (Figure 3c) and 2021 January 4 (Fig-

ure 3e) using the Three-hundred MilliMeter (300 mm)

Telescope (TMMT; Monson et al. 2017) at Las Cam-

panas Observatory in Chile. TMMT is a f/7.8 FRC300

from Takahashi on a German equatorial AP1600 GTO

mount with an Apogee Alta U42-D09 CCD Camera, FLI

ATLAS focuser, and Centerline filter wheel.

On 2020 December 15, the target rose from an airmass

of 1.86 at the start of the observations to a minimum

airmass of 1.32 and then set to an airmass of 2.62 at

the end of the observations. On 2021 January 4, the

target rose from an airmass of 1.48 to a minimum air-

mass of 1.32 and then set to an airmass of 3.16 at the

end of observations. Observations on both nights were

performed using the Bessell I filter with 1 × 1 on-chip

binning and exposure times of 120 s. In the 1 × 1 bin-

4 Kanodia et al. 2021.

85°03' 04' 05' 06' 07'

11°35'

34'

33'

32'

Right Ascension

Dec

linat

ion

(b) ZTF (zr) Image from 2019.0

85°03' 04' 05' 06' 07'

11°35'

34'

33'

32'

Right Ascension

Dec

linat

ion

(a) POSS-I (Red) Image from 1951.8TOI-532

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

∆ M

ean

GR

P M

ag

Figure 1. Panel a overlays the 11 x 11 pixel TESS Sector 6 footprint (blue grid) on a POSS-I red image from 1951.8. TOI-532does not have significant proper motion, as can be seen while comparing Panel a) and b). The TESS aperture is outlined inred and we highlight our target TOI-532. No bright targets are present inside the TESS aperture with ∆ GRP < 3. Panel bis similar to Panel A but with a background image from ZTF zr (5600 A– 7316 A) 2019 (Masci et al. 2019).

Figure 2. Short cadence (2 minute) time series TESS PDCSAP photometry (without detrending) from Sector 6, with thebinned data (in 1 hour bins), along with the TOI-532b transits overlaid in blue.

ning mode, TMMT has a gain of 1.35 e/ADU, a plate

scale of 1.194 ′′/pixel, and a readout time of 6 s.

In addition to the standard corrections, a fringe sub-

traction was also performed for the TMMT I band im-

ages. The final light curve from 2020 December 15 uti-

lized a photometric aperture of 5 pixels (5.97′′), an inner

sky annulus of 20 pixels (23.9′′), and a outer sky annu-

lus of 30 pixels (35.8′′). The final light curve from 2021

January 4 utilized a photometric aperture of 5 pixels

(5.97′′), an inner sky annulus of 15 pixels (17.9′′) and

outer sky annulus of 30 pixels (35.8′′).

2.2.3. LCRO

We observed a transit of TOI-532b on the night of

2021 January 4 (Figure 3d) using the 305 mm Las Cam-

panas Remote Observatory (LCRO) telescope at the Las

Campanas Observatory in Chile. The LCRO telescope

is an f/8 Maksutov-Cassegrain from Astro-Physics on a

German Equatorial AP1600 GTO mount with an FLI

A super Neptune orbiting TOI-532 5

Figure 3. Photometric observations for TOI-532b; a) the TESS phased plot shows the light curve phase-folded to the best fitorbital period, b-f) Ground based observations for TOI-532b. The raw photometery is shown in grey, whereas in red we showthe photometry binned to 5 minute bins. The best-fit transit solution, along with the 1 σ confidence interval are shown in blue.

Proline 16803 CCD Camera, FLI ATLAS focuser and

Centerline filter wheel.

The target rose from an airmass of 1.40 at the start

of the observations to a minimum airmass of 1.32 and

then set to an airmass of 3.29 at the end of the observa-

tions. Observations were performed using the SDSS i′

filter with 1 × 1 on-chip binning and an exposure time

of 240 s. In the 1 × 1 binning mode, LCRO has a gain

of 1.52 e/ADU, and a plate scale of 0.773 ′′/pixel, and a

readout time of 17 s. For the final reduction, we selected

a photometric aperture of 6 pixels (4.64′′) with an inner

sky annulus of 13 pixels (10.0′′) and outer sky annulus

of 30 pixels (23.2′′).

2.2.4. Diffuser-assisted Photometry with the 3.5m ARCTelescope

We observed a transit of TOI-532b (Figure 3f) on the

night of 2021 February 1 using the 3.5 m Astrophysical

Research Consortium (ARC) Telescope Imaging Camera

(ARCTIC; Huehnerhoff et al. 2016) at the ARC 3.5m

Telescope at Apache Point Observatory (APO). We ob-

served the transit using the Engineered Diffuser avail-

able on ARCTIC, which we designed to enable precision

photometric observations from the ground on nearby

bright stars (Stefansson et al. 2017).

The target set from an airmass of 1.07 at the start of

the observations to an airmass of 1.14 at the end of the

observations. The observations were performed using

the SDSS i′ filter with an exposure time of 20 s in the

LL-readout and fast readout modes with 4 × 4 on-chip

binning. In the 4 × 4 binning mode, ARCTIC has a

gain of 2.0 e/ADU, a plate scale of 0.456 ′′/pixel, and

a readout time of 2.7 s. Due to cloud contamination,

only the egress of the transit was recovered from the

data. For the final reduction, we selected a photometric

aperture of 13 pixels (5.72′′), an inner sky annulus of

30 pixels (13.2′′), and outer sky annulus of 45 pixels

(19.8′′).

2.3. High Contrast Imaging

2.3.1. ShARCS on the Shane telescope

We observed TOI-532 using the ShARCS camera on

the Shane 3m telescope at Lick Observatory (Srinath

et al. 2014). Due to instrument repairs, we were unable

to use the Laser Guide Star (LGS) mode, and had to

use Natural Guide Star (NGS) mode. This mode can

be more challenging for faint targets, as the guider cam-

era can easily lose the target, but conditions were good

enough to retrieve data for TOI-532. The target was

observed using a 5 point dither process as outlined in

Furlan et al. (2017).

6 Kanodia et al. 2021.

Table 1. Summary of ground based photometric follow up of TOI-532

Obs Date Filter Exposure PSF Apertures: Photometric, Field of View

(YYYY-MM-DD) Time (s) FWHM (”) Inner, Outer Annuli (”) (’)

RBO (0.6 m)

2020-12-07 Bessell I 120 8.88 (Defocus) 9.04, 21.3, 31.9 8.94 × 8.94

TMMT (0.3 m)

2020-12-15 Bessell I 120 3.49 5.97, 23.9, 35.8 40.75 × 40.75

2021-01-04 Bessell I 120 3.18 5.97, 17.9, 35.8 40.75 × 40.75

LCRO (0.3 m)

2021-01-04 i’ 240 2.45 4.64, 10.0, 23.2 51.97 × 51.97

APO (3.5 m)

2021-02-01 i’ 20 7.67 (Diffusera) 5.72, 13.2, 19.8 7.9 × 7.9

aEngineered diffuser with 8.7′′ FWHM (Stefansson et al. 2017)

Figure 4. 5σ contrast curve for TOI-532 observed fromNESSI in the Sloan r′ and z′ filters showing no bright com-panions within 1.2′′ from the host star. The z′ image isshown as an inset 1′′ across.

The data is then reduced using a custom AO pipeline

developed internally. This pipeline first rejects all over-

exposed or underexposed images, and we then manu-

ally exclude data we know to be erroneous (lost guiding

on the star, shutters closed early due to weather, etc.).

Next we apply a standard dark correction, flat correc-

tion, and sigma clipping process. A master sky image

is produced from the 5 point dither process, and sub-

tracted from each image. A final image is then produced

using an interpolation process to shift the images onto

a single centroid.

Finally, we use the algorithm developed by Espinoza

et al. (2016) to generate a 5 sigma contrast curve as a

part of the final analysis (Figure 5). We detected no

Figure 5. 5σ contrast curve for TOI-532 from the ShARCScamera on the Shane 3 m telescope. We detected no com-panions within 0.507 ± 0.017 ′′ corresponding to a ∆Ks of3.7. The inset shows a 10 ′′ region around the star.

companions within > 0.507 arcseconds corresponding to

a ∆Ks of 3.7.

2.3.2. NESSI at WIYN

We supplement our AO data with speckle imaging

observations taken on 2021 April 3 using the NN-

Explore Exoplanet Stellar Speckle Imager (NESSI) on

the WIYN 3.5m telescope at Kitt Peak National Obser-

vatory. To search for faint background stars and stellar

companions, we collected a 9 minute sequence of 40 ms

diffraction-limited exposures of TOI-532 with the Sloan

r′ and z′ filters. As we show in Figure 4, the NESSI data

show no evidence of blending from a bright companion

at separations > 0.15′′at ∆r′ = 3.1, and ∆z′ = 3.5.

A super Neptune orbiting TOI-532 7

Figure 6. Time series of RV observations of TOI-532 with HPF. The best-fitting model derived from the joint fit to thephotometry and RVs is plotted in blue, including the 16-84% confidence interval in light blue. The bottom panel shows theresiduals after subtracting the model. The jitter that is added in quadrature to the HPF errorbars (is shown in red), and isnegligible compared to the HPF errorbars.

2.4. Radial Velocity Follow-up with the Habitable-zone

Planet Finder

We observed TOI-532 using HPF (Mahadevan et al.

2012, 2014), a near-infrared (8080− 12780 A), high res-

olution precision RV spectrograph located at the 10 me-

ter Hobby-Eberly Telescope (HET) in Texas. HET is

a fixed-altitude telescope with a roving pupil design.

It is fully queue-scheduled telescope with all observa-

tions executed in a queue by the HET resident as-

tronomers (Shetrone et al. 2007). HPF is a fiber-fed in-strument with a separate science, sky and simultaneous

calibration fiber (Kanodia et al. 2018), and is actively

temperature-stabilized at the milli-Kelvin level (Stefans-

son et al. 2016). We use the algorithms described in

the tool HxRGproc for bias removal, non-linearity cor-

rection, cosmic ray correction, slope/flux and variance

image calculation (Ninan et al. 2018) of the raw HPF

data. HPF has the capability for simultaneous calibra-

tion using a NIR Laser Frequency Comb (LFC; Metcalf

et al. 2019), however owing to the faintness of our target

we chose to avoid simultaneous calibration to minimize

the impact of scattered calibrator light in the science

target spectra. Instead, we interpolate the wavelength

solution from other LFC exposures on the night of the

observations, to correct for the well calibrated instru-

ment drift, as has been discussed in Stefansson et al.

(2020). This method has been shown to enable precise

wavelength calibration and drift correction with a preci-

sion of ∼ 30 cm/s per observation, a value much smaller

than our estimated per observation RV uncertainty (in-

strumental + photon noise) for this object of ∼ 22 m/s

(in 649 s exposures).

To estimate the RVs, we follow the method de-

scribed in Stefansson et al. (2020), by using a modified

version of the SpEctrum Radial Velocity AnaLyser

pipeline (SERVAL; Zechmeister et al. 2018). SERVAL uses

the template-matching technique to derive RVs (e.g.,

Anglada-Escude & Butler 2012), where it creates a mas-

ter template from the target star observations, and de-

termines the Doppler shift for each individual observa-

tion by minimizing the χ2 statistic. We create this mas-

ter template by using all the HPF observations of TOI-

532, where telluric and sky-emission lines are masked in

the calculations of the RVs. The telluric regions iden-

tified by a synthetic telluric-line mask generated from

telfit (Gullikson et al. 2014), a Python wrapper to the

Line-by-Line Radiative Transfer Model package (Clough

et al. 2005). Given the faintness of our target, we do not

subtract out the sky fiber spectra from the sky fiber,

as we observed that doing so added additional read

noise, resulting in less precise RV measurements. To

perform our barycentric correction, we use barycorrpy,

the Python implementation (Kanodia & Wright 2018)

of the algorithms from Wright & Eastman (2014). We

8 Kanodia et al. 2021.

Table 2. HPF RVs of TOI-532.We include this table in a ma-chine readable format along withthe manuscript.

BJDTDB RV σ

(d) ( m/s) ( m/s)

2459159.82217 -28.41 12.02

2459179.76468 72.25 16.71

2459209.85498 33.54 20.41

2459210.67441 -7.51 21.93

2459216.82807 11.79 12.70

2459233.61084 35.41 17.17

2459237.77469 48.57 22.57

2459245.75139 -27.58 19.35

2459246.74596 54.53 19.16

2459247.75757 23.59 20.39

2459248.74084 25.75 13.16

2459265.69221 58.13 19.00

2459266.69484 -27.78 18.47

2459267.68911 49.75 16.90

2459270.68444 84.04 17.46

2459271.68015 -5.66 22.22

2459292.62161 -33.47 15.94

2459294.61636 -30.69 15.83

obtained a total of 19 visits on this target between 2020

November 5 and 2021 March 21, of which 1 visit was dis-

carded due to bad weather conditions. Each visit was

divided into 3 exposures of 649 seconds each, where the

median S/N of each HPF exposure was 37 per resolution

element. The individual exposures were then binned af-

ter weighting, with the final binned RVs being listed in

Table 2 and plotted in Figure 6.

3. STELLAR PARAMETERS

3.1. Spectroscopic Parameters with HPF-SpecMatch

Using the method described in Stefansson et al.

(2020), we use the HPF spectra to estimate the Teff ,

log g, and [Fe/H] values of the host star. This is based on

SpecMatch-Emp algorithm from Yee et al. (2017), where

we compare the high resolution HPF spectra of TOI-532

to a library of high S/N as-observed HPF spectra, which

consists of slowly-rotating reference stars with well char-

acterized stellar parameters from Yee et al. (2017).

We shift the observed target spectrum to a library

wavelength scale and rank all of the targets in the library

using a χ2 goodness-of-fit metric. After this initial χ2

minimization step, we pick the five best matching ref-

erence spectra (in this case: BD+29 2279, GJ 134, GJ

205, HD 28343, HD 88230) to construct a weighted spec-

trum using their linear combination to better match to

the target spectrum (Jones et al. 2021 in prep.). In

this step, each of the five stars receives a best-fit weight

coefficient. We then assign the target stellar parameter

Teff , log g, and [Fe/H] values as the weighted average of

the five best stars using the best-fit weight coefficients.

Our final parameters are listed in Table 3, and are de-

rived from the HPF order spanning 8670 – 8750 A. As

an additional check, we performed a similar library com-

parison using 6 other HPF orders which have low telluric

contamination, and obtain consistent stellar parameters

across them. Our error estimates are obtained from us-

ing the cross-validation method, as described by Stefans-

son et al. (2020). During both optimization steps, we ac-

count for any potential v sin i broadening by artificially

broadening the library spectra with a v sin i broadening

kernel (Gray 1992) to match the rotational broadening

of the target star. For TOI-532, we did not need sig-

nificant rotational broadening, and therefore place an

upper limit of v sin i < 2 km/s, which is the lower limit

of measurable v sin i values given HPF’s spectral resolv-

ing power of R ∼ 55, 000.

3.2. Model-Dependent Stellar Parameters

In addition to the spectroscopic stellar parameters de-

rived above, we use EXOFASTv2 (Eastman et al. 2013) to

model the SED of TOI-532 to derive model-dependent

constraints on the stellar mass, radius, and age of the

star. For the spectral energy distribution (SED) fit,

EXOFASTV2 uses the BT-NextGen stellar atmospheric

models (Allard et al. 2012). We assume Gaussian priors

on the (i) 2MASS JHK magnitudes, (ii) SDSS g′r′i′ and

Johnson B and V magnitudes from APASS, (iii) Wide-

field Infrared Survey Explorer magnitudes W1, W2, and

W3, (Wright et al. 2010), (iv) spectroscopically-derived

host star effective temperature, surface gravity, and

metallicity, and (v) distance estimate from Bailer-Jones

et al. (2021). We apply a uniform prior on the visual

extinction and place an upper limit using estimates of

Galactic dust by Green et al. (2019) (Bayestar19) calcu-

lated at the distance determined by Bailer-Jones et al.

(2021). We convert the Bayestar19 upper limit to a vi-

sual magnitude extinction using the Rv = 3.1 reddening

law from Fitzpatrick (1999).

We use GALPY (Bovy 2015) to calculate the UVW ve-

locities in the barycentric frame2, which along with the

2 With U towards the Galactic center, V towards the direction ofGalactic spin, and W towards the North Galactic Pole (Johnson& Soderblom 1987).

A super Neptune orbiting TOI-532 9

Table 3. Summary of stellar parameters for TOI-532.

Parameter Description Value Reference

Main identifiers:

TOI TESS Object of Interest 532 TESS mission

TIC TESS Input Catalogue 144700903 Stassun

2MASS · · · J05401918+1133463 2MASS

WISE · · · J054019.20+113345.6 WISE

Gaia EDR3 · · · 3340265717587057536 Gaia EDR3

Equatorial Coordinates, Proper Motion and Spectral Type:

αJ2016 Right Ascension (RA, degrees) 85.08005702(4) Gaia EDR3

δJ2016 Declination (Dec, degrees) 11.562632056(3) Gaia EDR3

µα Proper motion (RA, mas/yr) 23.24± 0.02 Gaia EDR3

µδ Proper motion (Dec, mas/yr) −38.04± 0.01 Gaia EDR3

d Distance in pc 134.61± 0.36 Bailer-Jones

AV,max Maximum visual extinction 0.01 Green

Optical and near-infrared magnitudes:

B Johnson B mag 15.769± 0.159 APASS

V Johnson V mag 14.395± 0.056 APASS

g′ Sloan g′ mag 15.136± 0.069 APASS

r′ Sloan r′ mag 13.802± 0.065 APASS

i′ Sloan i′ mag 13.068± 0.074 APASS

T TESS magnitude 12.678± 0.007 Stassun

J J mag 11.466± 0.023 2MASS

H H mag 10.749± 0.024 2MASS

Ks Ks mag 10.587± 0.025 2MASS

W1 WISE1 mag 10.488± 0.022 WISE

W2 WISE2 mag 10.541± 0.021 WISE

W3 WISE3 mag 10.436± 0.089 WISE

Spectroscopic Parametersa:

Teff Effective temperature in K 3957± 69 This work

[Fe/H] Metallicity in dex 0.38± 0.04 This work

log(g) Surface gravity in cgs units 4.67± 0.12 This work

Model-Dependent Stellar SED and Isochrone fit Parametersb:

Teff Effective temperature in K 3927± 37 This work

[Fe/H] Metallicity in dex 0.338+0.072−0.066 This work

log(g) Surface gravity in cgs units 4.669+0.018−0.017 This work

M∗ Mass in M� 0.639± 0.023 This work

R∗ Radius in R� 0.612+0.013−0.012 This work

L∗ Luminosity in L� 0.0803+0.0019−0.0018 This work

ρ∗ Density in g/cm3 3.92+0.22−0.21 This work

Age Age in Gyrs 7.1+4.4−4.8 This work

Other Stellar Parameters:

v sin i∗ Rotational velocity in km/s < 2km/s This work

∆RV “Absolute” radial velocity in km/s 9.67± 0.08 This work

U, V,W Galactic velocities (Barycentric) in km/s −2.22± 0.08,−30.20± 0.11,−1.24± 0.01 This work

U, V,W c Galactic velocities (LSR) in km/s 8.89± 0.72,−17.96± 0.48, 6.01± 0.36 This work

References are: Stassun (Stassun et al. 2018), 2MASS (Cutri et al. 2003), Gaia EDR3 (Gaia Collaboration et al. 2020),Bailer-Jones (Bailer-Jones et al. 2018), Green (Green et al. 2019), APASS (Henden et al. 2018), WISE (Wright et al.2010)

aDerived using the HPF spectral matching algorithm from Stefansson et al. (2020)

b EXOFASTv2 derived values using MIST isochrones with the Gaia parallax and spectroscopic parameters in a) as priors.

cThe barycentric UVW velocities are converted into local standard of rest (LSR) velocities using the constants fromSchonrich et al. (2010).

10 Kanodia et al. 2021.

Figure 7. HPF RV observations phase folded on the bestfit orbital period from the joint fit from Section 4. The bestfitting model is shown in the solid line, whereas the 1σ con-fidence intervals are shown in blue.

BANYAN tool (Gagne et al. 2018) classify TOI-532 as

a field star in the thin disk with very high probability

(Bensby et al. 2014).

3.3. Estimating Rotation Period

We note that the TESS photometry (PDCSAP un-

detrended photometry shown in Figure 2) is relatively

flat, and shows no flaring activity. We also run a gen-

eralized Lomb Scargle (GLS) periodogram (Zechmeis-

ter & Kurster 2009) on the TESS photometry using its

astropy implementation, and find no significant peaks

with a False Alarm Probability 1%3. This is consistent

with an inactive star with a long rotation period.

4. JOINT FITTING OF PHOTOMETRY AND RVS

We perform a joint fit of all the photometry (TESS +

ground based sources), and the RVs using the Python

packge exoplanet, which uses PyMC3 the Hamiltonian

Monte Carlo (HMC) package (Salvatier et al. 2016).

The exoplanet package uses starry (Luger et al.

2019; Agol et al. 2020) to model the planetary tran-

sits, using the analytical transit models from Mandel &

Agol (2002), which includes a quadratic limb darkening

law. These limb darkening priors are implemented in

exoplanet using the reparameterization suggested by

Kipping (2013) for uninformative sampling. We fit each

phased transit shown in Figure 3 with separate limb

darkening coefficients4. In the photometric model we

include a dilution factor for the TESS photometry, D,

to represent the ratio of the out-of-transit flux of TOI-

532 to that of all the stars within the TESS aperture,

that has not been corrected for. We assume that the

3 The PDCSAP photometry from TESS flattens variability ontimescales longer than about 10 days (Jenkins et al. 2016), andtherefore our search using the TESS photometry is insensitive tostellar rotation periods longer than this.

4 We also try fitting the photometry with a single set of limb dark-ening coefficients for all the transits, and obtain similar results.

higher spatial resolution ground based photometry has

no dilution, since we use the ground based transits to

estimate the dilution in the TESS photometry. We as-

sume the transit depth is identical in all bandpasses and

use our ground-based transits to determine the dilution

required in the TESS data to be DTESS = 0.92 ± 0.06;

including which, gives us a radius of 5.82 ± 0.19 R⊕.

If the blending effects due to background stars are cor-

rectly accounted for by the SPOC pipeline, we expect

this dilution term to be close to 1.

We model the RVs using a standard Keplerian model.

We try an eccentric joint fit to the photometry and RVs,

and obtain an eccentricity consistent with a circular or-

bit at ∼ 1σ. Considering the Lucy-Sweeney bias (Lucy

& Sweeney 1971), we adopt a circular orbit by fixing the

eccentricity to 0, and the argument of periastron to 90◦.

For both the photometry and RV modeling, we include

a simple white-noise model in the form of a jitter term

that is added in quadrature to the error bars of each

data set.

We use scipy.optimize to find the initial maximum

a posteriori (MAP) parameter estimates, which are then

used as the initial conditions for parameter estimation

using ”No U-Turn Sampling” (NUTS, Hoffman & Gel-

man 2014), implemented for the HMC sampler PyMC3,

where we check for convergence using the Gelman-Rubin

statistic (R ≤ 1.1; Ford 2006). We also run a joint fit

using juliet (Espinoza et al. 2019), and verify that we

obtain fit parameters similar to those from exoplanet.

The host stellar density constrained from the transit

fit to the TESS photometry (Seager & Mallen-Ornelas

2003) is consistent with that obtained from the SED fit

for an M0 host star (Section 3.2). The final derived

planet parameters are shown in Table 4, and the phased

HPF RVs are shown in Figure 7. We obtain a mass for

TOI-532b of 61.5+9.7−9.3 M⊕, and a radius of 5.82 ± 0.19

R⊕.

5. DISCUSSION

5.1. Giant Planet Dependence on Host Star Metallicity

In Figure 8a we show TOI-532 b with respect to other

M dwarf exoplanets with mass measurements at 3σ or

higher. The data is taken from the NASA Exoplanet

Archive (Akeson et al. 2013), and includes recent M

dwarf transiting planets discovered by TESS. TOI-532b

has properties similar to three recent super Neptunes

discovered by TESS that orbit M dwarf stars - TOI-

1728b (Kanodia et al. 2020), LP 714-417b (TOI-442 b;

Dreizler et al. 2020), and TOI-674b (Murgas et al. 2021).

TOI-532b represents the largest and most massive Super

Neptune found orbiting an M dwarf.

A super Neptune orbiting TOI-532 11

Table 4. Derived Parameters for the TOI-532 System

Parameter Units Value

Orbital Parameters:

Orbital Period . . . . . . . . . . . . P (days) . . . . . . . . . . . . . 2.3266508±0.0000030

Eccentricity . . . . . . . . . . . . . . . e . . . . . . . . . . . . . . . . . . . . 0 (fixed)

Argument of Periastron . . . ω (degrees) . . . . . . . . . . 90 (fixed)

Semi-amplitude Velocity . . K (m/s) . . . . . . . . . . . . . . 39.82+6.15−5.98

Systemic Velocitya . . . . . . . . γ (m/s) . . . . . . . . . . . . . . 16.42+5.04−4.83

RV trend . . . . . . . . . . . . . . . . . dv/dt ( m/s/yr) 0.35+5.08−4.99

RV jitter . . . . . . . . . . . . . . . . . . σHPF (m/s). . . . . . . . . . . 11.43+6.62−8.84

Transit Parameters:

Transit Midpoint . . . . . . . . . TC (BJDTDB) . . . . . . . . 2458470.576777+0.000860−0.000902

Scaled Radius . . . . . . . . . . . . . Rp/R∗ . . . . . . . . . . . . . . . 0.0877± 0.0016

Scaled Semi-major Axis . . . a/R∗ . . . . . . . . . . . . . . . . 10.49+0.25−0.23

Orbital Inclination . . . . . . . . i (degrees) . . . . . . . . . . . . 88.08+0.51−0.41

Transit Duration . . . . . . . . . . T14 (days) . . . . . . . . . . . . 0.0728±0.001

Photometric Jitterb . . . . . . σTESS (ppm). . . . . . . . . 76+66−45

σRBO20201207 (ppm). . . 895+578−584

σTMMT20201215 (ppm) . 434+482−300

σLCRO20210104 (ppm). . 540+697−382

σTMMT20210104 (ppm) . 823+789−585

σARCTIC20210201 (ppm) 770+101−98

Dilutionc . . . . . . . . . . . . . . . . . DTESS . . . . . . . . . . . . . . . 0.92± 0.06

Planetary Parameters:

Mass. . . . . . . . . . . . . . . . . . . . . . Mp (M⊕) . . . . . . . . . . . . . 61.5+9.7−9.3

Radius . . . . . . . . . . . . . . . . . . . . Rp (R⊕) . . . . . . . . . . . . . 5.82± 0.19

Density . . . . . . . . . . . . . . . . . . . ρp (g/ cm3) . . . . . . . . . . . 1.72± 0.31

Semi-major Axis . . . . . . . . . . a (AU) . . . . . . . . . . . . . . 0.0296± 0.00035

Average Incident Fluxd . . . 〈F 〉 ( 105 W/m2) . . . . . 1.28±0.11

Planetary Insolation S (S⊕) . . . . . . . . . . . . . . . 94.1± 8.0

Equilibrium Temperaturee Teq (K) . . . . . . . . . . . . . . . 867± 18

aIn addition to the Absolute RV from Table 3.

b Jitter (per observation) added in quadrature to photometric instrument error.

cDilution due to presence of background stars in TESS aperture, not accounted forin the PDCSAP flux.dWe use a Solar flux constant = 1360.8 W/m2, to convert insolation to incident flux.

eWe assume the planet to be a black body with zero albedo and perfect energyredistribution to estimate the equilibrium temperature.

12 Kanodia et al. 2021.

a) Mass–Radius plane b) Metallicity–Radius plane

Figure 8. We show TOI-532b (circled) in different planet parameter space along with other M dwarf planets with massmeasurements at > 3σ. a) Mass–Radius plane for M dwarf planets. We include contours of density 1, 3, 10 g/cm3, wherethe markers are colour coded by Teff . b) The metallicity of the host stars for the same planets. We note that all four superNeptunes highlighted in this plot are orbiting metal-rich early type M dwarfs.

a) Radius–Period plane b) Radius–Insolation plane

Figure 9. We note the location of TOI-532b in the Neptune desert (Szabo & Kiss 2011) along with a sample of transitingexoplanets that have their masses measured. a) The sample in the Radius–Period plane is colour coded by the log10 insolation,where the M dwarf planets are solid whereas the rest are shown to be translucent. The nominal Neptune desert boundariesfrom Mazeh et al. (2016) are denoted with dashed lines. b) We show TOI-532 in the Radius–Insolation plane, where M dwarfplanets are coloured according to their Teff , with planets orbiting other spectral type host stars denoted in purple. We use theNeptune Desert boundaries from Mazeh et al. (2016) in Radius–Period plane to calculate similar boundaries in the Insolationplane assuming 1 M� and 1 L� (Solar type star) in black, and with 0.6 M� and 0.075 L� (M0 star) in grey for representativepurposes. Even though TOI-532 is placed in the middle of the Neptune desert in the Radius–Period plane, we note that in theRadius–Insolation plane it is placed by the edge of the desert, highlighting the importance of considering the insolation fluxesfor planetary evolution.

A super Neptune orbiting TOI-532 13

TOI-532b orbits a metal-rich M star, similar to the

other gas giants found around M dwarfs (Figure 8b).

This positive metallicity correlation favours the core-

accretion formation mechanism (Pollack et al. 1996;

Schlaufman 2018); which can be explained if these gas

giants formed due to the collisions of 10 M⊕ cores (Pe-

tigura et al. 2018). The probability of formation of these

cores increases with metallicity, and therefore it should

be easier to form such gaseous planet cores around

metal-rich stars, before the protoplanetary disk depletes

(Ida & Lin 2004b). In-situ formation of these gas gi-

ants at such orbital periods (and hence orbital separa-

tions) also requires super-Solar metallicity protoplane-

tary disks to provide enough material for the formation

of their cores (Dawson et al. 2015; Boley et al. 2016;

Batygin et al. 2016).

An alternative to the accretion theories of formation

, is gravitational instability (GI; Boss 1997). This has

been proposed to explain the formation of gas giants

around these low mass stars (Boss 2006), especially the

mid-to-late M dwarfs (Morales et al. 2019). The amount

of material available in these disks would be too little to

form cores that are massive enough to accrete gaseous

envelopes from the disk before it gets depleted (Laughlin

et al. 2004), lending credibility to GI as a potential for-

mation mechanism. The discovery of gas giants such as

TOI-532b, adds to the sparse population of these objects

around M dwarfs, which can ultimately help differenti-

ate between these two competing theories.

5.2. Neptune desert

Figure 9 shows the Neptune desert which is character-

ized by a dearth of planets. We highlight the location of

TOI-532b in the Neptune desert (Mazeh et al. 2016) in

the Radius–Period plane (Figure 9a), where it falls in the

middle of this region. The figure includes transiting exo-

planets with mass measurements, coloured according to

their insolation, with the M dwarf planets shown as solid

markers, whereas those orbiting other spectral types

hosts are translucent. Different processes have been pro-

posed to explain this feature, which include photoevap-

oration (Owen & Lai 2018; Ionov et al. 2018), and high

eccentricity migration (Matsakos & Konigl 2016).

Although typically parameterized in terms of orbital

period, it is important to consider that in a combined

sample of FGK and M dwarf host stars, the bolometric

insolation can differ by more than an order of magnitude

for similar orbital separations (e.g., between a G type

host, and an early M dwarf). McDonald et al. (2019)

suggest that this variation in the bolometric luminos-

ity is the primary reason for the discrepancies in the

location of the Neptune desert as a function of spectral

type. Therefore, we also plot TOI-532 in the Radius–

Insolation plane (Figure 9b), and include the desert

boundaries from Mazeh et al. (2016) which were esti-

mated using a predominantly FGK planet sample. We

include these in the Insolation–Radius plane assuming

a Solar mass and luminosity, and also assuming an M0

host star. While TOI-532 is located inside the Neptune

desert in the Radius–Period plane, when accounting for

the incident insolation, it is located by the edge of this

desert. We therefore suggest that in order to compare a

sample of planets across spectral types FGK, and M, the

Neptune desert should be characterized in the Radius–

Insolation plane.

The under-density of planets in this highly irradiated

region has often been attributed to atmospheric escape

due to photoevaporation (Owen & Lai 2018). The rate

and efficacy of photoevaporation is highly dependent on

the X-ray and ultraviolet flux (XUV) from the host star;

where a planet around a mid-type M dwarf can receive

100x more integrated X-ray flux than a solar type star

(for the same insolation). When the frequency distribu-

tion of these gaseous planets is considered as a function

of lifetime integrated X-ray flux, most of the variabil-

ity between spectral types is accounted for (McDonald

et al. 2019).

Characterization of planets such as TOI-532b, which

lie within the Neptune desert, can help provide con-

straints on the potential formation mechanisms respon-

sible for clearing out the Neptune desert. Estimating

the fraction of H/He within its atmosphere would help

bound the extent of photoevaporation, and its role in

sculpting this desert. TOI-532b helps increase the small

sample of planets situated inside this desert. Comparing

the stellar (metallicity, age, stellar mass) and planetary

parameters (density, planetary mass) for the sample of

planets inside the desert to the larger exoplanet sample

can help highlight potential formation mechanisms; per-

haps as an extension to the radius valley (Fulton et al.

2017), and it’s dependence on various stellar properties

(Owen & Murray-Clay 2018; Cloutier et al. 2019; Berger

et al. 2020; Van Eylen et al. 2021).

5.3. Planetary Composition and Photoevaporation

We use the giant planet models from Fortney et al.

(2007) to estimate a core mass of ∼ 36 M⊕ for TOI-532b,

corresponding to an atmospheric mass (H/He) fraction

of ∼ 25%. Super Neptunes such as TOI-532b present an

intermediate population of gaseous planets between sub-

Neptunes (Bean et al. 2021) and Jovian planets (Mor-

dasini et al. 2016; Dawson & Johnson 2018). A subset

of these Super Neptunes with equilibrium temperatures

between 800–1200 K span the range where models pre-

14 Kanodia et al. 2021.

dict a transition from methane dominated atmospheres

to carbon monoxide (Guzman-Mesa et al. 2020). Char-

acterizing the atmospheres of planets such as TOI-532b

with equilibrium temperatures of ∼ 850 K by constrain-

ing their C/H and C/O ratios, can help place constraints

on their formation history as well as atmospheric chem-

istry (Madhusudhan et al. 2017).

TOI-532 is relatively faint (J = 11.46), but is still ac-

cessible from 10-m class telescopes (Tamura et al. 2012;

Kotani et al. 2018), as a potential target for detect-

ing atmospheric escape using the He 10830 A triplet.

Considering the small number of suitable targets for

such a measurement, we discuss the possibility of de-

tecting atmospheric escape from TOI-532b. It is use-

ful to compare TOI-532b to a similar planet with such

a detection—GJ 3470b (Ninan et al. 2020; Palle et al.

2020)—and also to a planet without a He 10830 A de-

tection, TOI-1728b (Kanodia et al. 2020). In the en-

ergy limited mass outflow regime5, the exosphere out-

flow is proportional to the irradiated extreme ultra vio-

let (EUV) flux and inversely proportional to the planet

density. TOI-532 is an earlier M0 star than the M1.5 GJ

3470, with its spectral type more favourable with higher

EUV radiation. However, TOI-532 is an older (and qui-

eter) 7.2+4.6−4.7 Gyr star, while GJ 3470 is relatively young

at ∼ 3 Gyr6. If we consider the EUV flux from the host

star to be similar, due to the larger radius of the host

star, the EUV irradiance on TOI-532b is 1.6 times that

of GJ 3470b, which can make up for the 1.8 times higher

density of TOI-532b than GJ 3470b. Thus, if the EUV

flux of TOI-532 (7 Gyr, M0) is similar to GJ 3470 (3 Gyr,

M1.5), we could expect a similar exosphere evaporation

and mass outflow in TOI-532b like in GJ 3470b. Under

this condition, He 10830 A absorption during transit is

a good probe to detect any signatures of outflow from

TOI-532.

Conversely, the other planet TOI-1728b has a host

star very similar to TOI-532 in both spectral type and

age. TOI-532 orbits 1.25 times closer than TOI-1728b,

and it is 1.5 times denser than TOI-1728b. Therefore,

from a simple scaling relationship we expect the mass

outflows in them to be only slightly less or very similar.

That being said, TOI-1728b had a null detection of He

10830 A with an upper limit of 1.1% (Kanodia et al.

5 The energy limited regime is a reasonable assumption here sincethe gravitational potential of this planet is 12.81 erg g−1 (log10(GM/R); Salz et al. 2016) . This is not a system with a lowdensity upper-atmosphere, like those seen in planets with highergravitational potential (>13.3 erg g−1)

6 GJ 3470b stellar and planetary parameters are from Kosiareket al. (2019).

2020). We therefore note that though the planetary pa-

rameters are amenable, the plausibility of a detectable

outflow from this super Neptune hinges on the EUV ir-

radiation environment of the host star.

6. SUMMARY

In this work, we report the discovery and confirma-

tion of a super Neptune, TOI-532b, orbiting an M0 star

in a ∼ 2.3 day circular orbit. We detail the TESS pho-

tometry, ground-based follow-up photometry, high con-

trast imaging, and also the RV observations performed

using HPF. Furthermore, we discuss how the planet

is situated at the edge of the Neptune desert in the

Radius–Insolation plane, and discuss potential for He

10830 A absorption detection using transmission spec-

troscopy. We also discuss the metallicity correlation for

gas giants occurrence, and how it continues down to the

M dwarf spectral type.

The discovery and mass measurement of gas giants

such as TOI-532b adds to the small sample of such plan-

ets around M dwarf host stars, and can help inform the-

ories of planetary formation and evolution. Therefore

we encourage future observations to place limits on at-

mospheric escape using the He 10830 A transition.

7. ACKNOWLEDGEMENTS

This research made use of Lightkurve, a Python pack-

age for Kepler and TESS data analysis (Lightkurve Col-

laboration, 2018).

This paper is based on observations obtained from

the Las Campanas Remote Observatory that is a part-

nership between Carnegie Observatories, The Astro-

Physics Corporation, Howard Hedlund, Michael Long,

Dave Jurasevich, and SSC Observatories.

This work has made use of data from the Euro-

pean Space Agency (ESA) mission Gaia (https://www.

cosmos.esa.int/gaia), processed by the Gaia Data Pro-

cessing and Analysis Consortium (DPAC, https://www.

cosmos.esa.int/web/gaia/dpac/consortium). Funding

for the DPAC has been provided by national institu-

tions, in particular the institutions participating in the

Gaia Multilateral Agreement.

The Center for Exoplanets and Habitable Worlds is

supported by the Pennsylvania State University, the

Eberly College of Science, and the Pennsylvania Space

Grant Consortium. These results are based on observa-

tions obtained with the Habitable-zone Planet Finder

Spectrograph on the HET. We acknowledge support

from NSF grants AST 1006676, AST 1126413, AST

1310875, AST 1310885, and the NASA Astrobiology In-

stitute (NNA09DA76A) in our pursuit of precision ra-

dial velocities in the NIR. We acknowledge support from

A super Neptune orbiting TOI-532 15

the Heising-Simons Foundation via grant 2017-0494.

The Hobby-Eberly Telescope is a joint project of the

University of Texas at Austin, the Pennsylvania State

University, Ludwig-Maximilians-Universitat Munchen,

and Georg-August Universitat Gottingen. The HET is

named in honor of its principal benefactors, William P.

Hobby and Robert E. Eberly. The HET collaboration

acknowledges the support and resources from the Texas

Advanced Computing Center. We thank the Resident

astronomers and Telescope Operators at the HET for

the skillful execution of our observations with HPF.

We acknowledge support from NSF grants AST-

1909506 and AST-1907622 and the Research Cor-

poration for precision photometric observations with

diffuser-assisted photometry.

This work was performed under the following finan-

cial assistance award 70NANB18H006 from U.S. Depart-

ment of Commerce, National Institute of Standards and

Technology

This research has made use of the NASA Exoplanet

Archive, which is operated by the California Institute

of Technology, under contract with the National Aero-

nautics and Space Administration under the Exoplanet

Exploration Program. This work includes data collected

by the TESS mission, which are publicly available from

MAST. Funding for the TESS mission is provided by

the NASA Science Mission directorate. Some of the

data presented in this paper were obtained from MAST.

Support for MAST for non-HST data is provided by the

NASA Office of Space Science via grant NNX09AF08G

and by other grants and contracts.

This research has made use of the SIMBAD database,

operated at CDS, Strasbourg, France, and NASA’s As-

trophysics Data System Bibliographic Services.

Some of the observations in this paper made use of

the NN-EXPLORE Exoplanet and Stellar Speckle Im-

ager (NESSI). NESSI was funded by the NASA Exo-

planet Exploration Program and the NASA Ames Re-

search Center. NESSI was built at the Ames Research

Center by Steve B. Howell, Nic Scott, Elliott P. Horch,

and Emmett Quigley.

Part of this research was carried out at the Jet Propul-

sion Laboratory, California Institute of Technology, un-

der a contract with the National Aeronautics and Space

Administration (NASA).

Computations for this research were performed on

the Pennsylvania State University’s Institute for Com-

putational and Data Sciences Advanced CyberInfras-

tructure (ICDS-ACI), including the CyberLAMP clus-

ter supported by NSF grant MRI-1626251. This work

includes data from 2MASS, which is a joint project of

the University of Massachusetts and IPAC at Caltech

funded by NASA and the NSF. CIC acknowledges sup-

port by NASA Headquarters under the NASA Earth

and Space Science Fellowship Program through grant

80NSSC18K1114. SK would like to acknowledge Monae

and Theodora for help with this project.

This research made use of exoplanet (Foreman-Mackey

et al. 2021a) and its dependencies (Agol et al. 2020; Ku-

mar et al. 2019; Robitaille et al. 2013; Astropy Collabo-

ration et al. 2018; Kipping 2013; Luger et al. 2019; The

Theano Development Team et al. 2016; Salvatier et al.

2016; Foreman-Mackey et al. 2021b)

Facilities: Gaia, HET (HPF), TESS, TMMT,

LCRO, RBO, APO (ARCTIC), WIYN (NESSI), Shane

(ShARCS), Exoplanet Archive

Software: ArviZ (Kumar et al. 2019), AstroIm-

ageJ (Collins et al. 2017), astroquery (Ginsburg et al.

2019), astropy (Robitaille et al. 2013; Astropy Collab-

oration et al. 2018), barycorrpy (Kanodia & Wright

2018), HxRGproc (Ninan et al. 2018), ipython (Perez

& Granger 2007), juliet (Espinoza et al. 2019),

lightkurve (Lightkurve Collaboration et al. 2018),

matplotlib (Hunter 2007), MRExo (Kanodia et al.

2019), numpy (Oliphant 2006), pandas (McKinney 2010),

PyMC3(Salvatier et al. 2016), scipy (Oliphant 2007; Vir-

tanen et al. 2020), SERVAL (Zechmeister et al. 2018),

starry (Luger et al. 2019; Agol et al. 2020), Theano (The

Theano Development Team et al. 2016).

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